Spectral purity filters for use in a lithographic apparatus

ABSTRACT

According to an aspect of the present invention, a spectral purity filter includes an aperture, the aperture being arranged to diffract a first wavelength of radiation and to allow at least a portion of a second wavelength of radiation to be transmitted through the aperture, the second wavelength of radiation being shorter than the first wavelength of radiation, wherein the aperture has a diameter greater than 20 μm.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/500,198, filed Jul. 9, 2009 (that issued as U.S. Pat. No. 8,390,788on Mar. 5, 2013), which claims the benefit under 35 U.S.C. §119(e) ofU.S. Provisional Application No. 61/079,975, filed Jul. 11, 2008; U.S.Provisional Patent Application No. 61/136,150, filed Aug. 14, 2008; andU.S. Provisional Patent Application No. 61/136,983, filed Oct. 20, 2008,which are incorporated by reference herein in their entireties.

BACKGROUND

1. Field

Embodiments of the present invention relate to spectral purity filters(SPFs), and in particular, although not restricted to, spectral purityfilters for use in a lithographic apparatus.

2. Background

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In that instance, a patterning device, whichis alternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.,including part of, one, or several dies) on a substrate (e.g., a siliconwafer). Transfer of the pattern is typically via imaging onto a layer ofradiation-sensitive material (e.g., resist) provided on the substrate.In general, a single substrate will contain a network of adjacent targetportions that are successively patterned. Known lithographic apparatusinclude so-called steppers, in which each target portion is irradiatedby exposing an entire pattern onto the target portion at one time, andso-called scanners, in which each target portion is irradiated byscanning the pattern through a radiation beam in a given direction (the“scanning” direction) while synchronously scanning the substrateparallel or anti-parallel to this direction. It is also possible totransfer the pattern from the patterning device to the substrate byimprinting the pattern onto the substrate.

In order to be able to project ever smaller structures onto substrates,it has been proposed to use extreme ultraviolet radiation (EUV) having awavelength within the range of 5-20 nm, for example within the range of13-14 nm. It has further been proposed that radiation with a wavelengthof less than 10 nm could be used, for example 6.7 nm or 6.8 nm. In thecontext of lithography, wavelengths of less than 10 nm are sometimesreferred to as ‘beyond EUV’ or as ‘soft x-rays’.

Extreme ultraviolet radiation and beyond EUV radiation may be producedusing, for example, a plasma. The plasma may be created for example bydirecting a laser at particles of a suitable material (e.g., tin), or bydirecting a laser at a stream of a suitable gas or vapor, such as Xe gasor Li vapor. The resulting plasma emits extreme ultraviolet radiation(or beyond EUV radiation), which is collected using a collector such asa mirrored grazing incidence collector, which receives the extremeultraviolet radiation and focuses the radiation into a beam.

Practical EUV Sources, such those which generate EUV radiation using aplasma, do not only emit desired ‘in-band’ EUV radiation, but alsoundesirable ‘out-of-band’ radiation. This out-of-band radiation is mostnotably in the deep ultraviolet (DUV) radiation range (100-400 nm).Moreover, in the case of some EUV sources, for example laser producedplasma EUV sources, the radiation from the laser, usually at 10.6 μm,presents a significant amount of out-of-band radiation.

In a lithographic apparatus, spectral purity is required for severalreasons. One reason is that resist is sensitive to out-of-bandwavelengths of radiation, and thus the image quality of patterns appliedto the resist may be deteriorated if the resist is exposed to suchout-of-band radiation. Furthermore, out-of-band infrared radiation, forexample the 10.6 μm radiation in some laser produced plasma sources,leads to unwanted and unnecessary heating of the patterning device,substrate and optics within the lithographic apparatus. Such heating maylead to damage of these elements, degradation in their lifetime, and/ordefects or distortions in patterns projected onto and applied to aresist-coated substrate.

In order to overcome these problems, several different transmissivespectral purity filters have been proposed which substantially preventthe transmission of infrared radiation, whilst simultaneously allowingthe transmission of EUV radiation. Some of these proposed spectralpurity filters include a thin metal layer or foil which is substantiallyopaque to, for example, infrared radiation, while at the same time beingsubstantially transparent to EUV radiation. These and other spectralpurity filters may also be provided with one or more apertures. The sizeand spacing of the apertures may be chosen such that infrared radiationis diffracted by the apertures, while EUV radiation is transmittedthrough the apertures. A spectral purity filter provided with aperturesmay have a higher EUV transmittance than a spectral purity filter whichis not provided with apertures. This is because EUV radiation will beable to pass through an aperture more easily than it would through agiven thickness of metal foil or the like.

One problem associated with spectral purity filters provided withapertures is that the apertures are so small that the manufacturingoptions that are available to create the apertures are limited and/orexpensive. Furthermore, the small diameter of the apertures reduces themechanical robustness of the spectral purity filter.

In a lithographic apparatus it is desirable to minimize the losses inintensity of radiation which is being used to apply a pattern to aresist coated substrate. One reason for this is that, ideally, as muchradiation as possible should be available for applying a pattern to asubstrate, for instance to reduce the exposure time and increasethroughput. At the same time, it is desirable to minimize the amount ofundesirable (e.g., out-of-band) radiation that is passing through thelithographic apparatus and which is incident upon the substrate.

It is therefore an object of embodiments of the present invention toprovide an improved or alternative spectral purity filter. For example,it is an object of embodiments of the present invention to provide aspectral purity filter provided with at least one aperture, and which iseasier to manufacture, and/or is more mechanically robust than known orproposed spectral purity filters. It is also an object of embodiments ofthe present invention to provide alternative spectral purity filterarrangements. It is a further object of embodiments of the presentinvention to provide a spectral purity filter with improved suppressionof undesirable (e.g., out-of-band) radiation, such as infraredradiation.

BRIEF SUMMARY OF THE INVENTION

According to a first aspect of the present invention there is provided aspectral purity filter, including: an aperture, the aperture beingarranged to diffract a first wavelength of radiation and to allow atleast a portion of a second wavelength of radiation to be transmittedthrough the aperture, the second wavelength of radiation being shorterthan the first wavelength of radiation; wherein the aperture has adiameter greater than 20 μm.

According to a second aspect of the present invention there is provideda spectral purity filter, including: an aperture, the aperture beingarranged to diffract a first wavelength of radiation and to allow atleast a portion of a second wavelength of radiation to be transmittedthrough the aperture, the second wavelength of radiation being shorterthan the first wavelength of radiation; wherein a sidewall of theaperture is provided with a coating.

The coating may be arranged to absorb at least a portion of the firstwavelength of radiation. The coating may be arranged to inhibitreflection of at least a portion of the first wavelength of radiation.The coating may be arranged to promote reflection of at least a portionof the second wavelength. The coating may be arranged to inhibitdegradation or environmental damage to the aperture.

The aperture may have a diameter greater than 20 μm.

According to the first or second aspects of the present invention, theaperture may have a diameter greater than 20 μm and less than or equalto 200 μm.

According to the first or second aspects of the present invention, thespectral purity filter may be provided with a plurality of apertures.The spectral purity filter may be provided with a periodic array ofapertures. The spectral purity filter may be provided with an aperiodicarray of apertures.

According to a third aspect of the present invention there is provided aspectral purity filter, including: a first aperture having a firstdiameter, the first aperture being arranged to diffract a firstwavelength of radiation and to allow at least a portion of a secondwavelength of radiation to be transmitted through the aperture, thesecond wavelength of radiation being shorter than the first wavelengthof radiation; wherein the spectral purity filter is also provided with:a second aperture having a second diameter, the second diameter beingsmaller than the first diameter, the second diameter being small enoughto prevent diffraction of the first wavelength of radiation, whileallowing transmission of at least a portion of the second wavelength ofradiation.

The first aperture has a diameter greater than 20 μm. The first aperturemay have a diameter greater than 20 μm and less than or equal to 200 μm.The second aperture may have a diameter which is less than or equal tohalf of the wavelength of the first wavelength of radiation.

The spectral purity filter may be provided with a plurality of firstapertures. The spectral purity filter may be provided with a periodicarray of first apertures. The spectral purity filter may be providedwith an aperiodic array of first apertures.

The spectral purity filter may be provided with a plurality of secondapertures. The spectral purity filter may be provided with a periodicarray of second apertures. The spectral purity filter may be providedwith an aperiodic array of second apertures.

According to the first, second or third aspects of the presentinvention, material forming the spectral purity filter may besubstantially transparent to the transmission of the second wavelengthof radiation. Material forming the spectral purity filter may besubstantially opaque to the transmission of the first wavelength ofradiation. Material forming the spectral purity filter may be arrangedto absorb or reflect the first wavelength of radiation.

According to the third aspect of the present invention, material formingthe spectral purity filter may be substantially transparent to thetransmission of the first wavelength of radiation. The spectral purityfilter may be arranged such that a phase difference is introducedbetween radiation of the first wavelength that is arranged to passthrough the material and radiation of the first wavelength that isarranged to pass through (and be diffracted by) the first aperture. Thespectral purity filter may be configured such that destructiveinterference of a zero diffraction order of the first wavelength ofradiation takes place between radiation of the first wavelength that isarranged to pass through (and be diffracted by) the material andradiation of the first wavelength that is arranged to pass through andbe diffracted by the first aperture. The spectral purity filter may havea thickness which causes the destructive interference.

According to a fourth aspect of the present invention there is provideda spectral purity arrangement, including: a spectral purity filterprovided with one or more apertures, the one or more apertures beingarranged to transmit and diffract a first wavelength of radiation and toallow at least a portion of a second wavelength of radiation to betransmitted through the one or more apertures, the second wavelength ofradiation being shorter than the first wavelength of radiation; materialforming the spectral purity filter being substantially transparent tothe transmission of the first wavelength of radiation, the spectralpurity filter being configured such that destructive interference of azero diffraction order of the first wavelength of radiation takes placebetween radiation of the first wavelength that is arranged to passthrough the material and radiation of the first wavelength that isarranged to pass through the one or more apertures; the spectral purityarrangement further including: a structure provided with a furtheraperture, the spectral purity filter and the structure being arrangedrelative to one another such that at least a portion of the secondwavelength of radiation is able to pass through the spectral purityfilter and the further aperture that is provided in the structure, andwherein a spacing or diameter of the one or more apertures of thespectral purity filter is configured to ensure that less than 50% ofradiation of the first wavelength of radiation is able to pass throughthe further aperture that is provided in the structure.

The spectral purity filter may be configured such that a phasedifference is introduced between radiation of the first wavelength thatis arranged to pass through the material and radiation of the firstwavelength that is arranged to pass through the one or more apertures,in order to cause the destructive interference. The spectral purityfilter may have a thickness which causes the destructive interference.

The spacing or diameter of the one or more apertures of the spectralpurity filter may be configured to ensure that a first diffraction orderof the first wavelength of radiation is incident upon the structure andnot transmitted through the further aperture of the structure.

The structure may be a plate.

The structure may be at least a part of a radiation source, anillumination system, or a projection system. The structure may be atleast a part of radiation source, an illumination system, or aprojection system of a lithographic apparatus. The structure may be atleast a part of a housing of the radiation source, at least a part of ahousing of the illumination system, or at least a part of a housing ofthe projection system.

The spectral purity filter may be provided with a plurality ofapertures. The spectral purity filter may be provided with a periodicarray of apertures or an aperiodic array of apertures.

The spacing or diameter of the one or more apertures of the spectralpurity filter may be configured to ensure that less than 10% ofradiation of the first wavelength of radiation is able to pass throughthe further aperture that is provided in the structure, or such thatless than 5% of radiation of the first wavelength of radiation is ableto pass through the further aperture that is provided in the structure.

According to the first, second, third or fourth aspects of the presentinvention, the first wavelength of radiation may have a wavelength thatis in the infrared part of the electromagnetic spectrum. The firstwavelength of radiation may have a wavelength that is approximately 10.6μm. The second wavelength of radiation may have a wavelength that is in,or is shorter than, the EUV part of the electromagnetic spectrum.

According to a fifth aspect of the present invention, there is provideda lithographic apparatus provided with a spectral purity filter orspectral purity arrangement according to the first, second, third orfourth aspects of the present invention.

The lithographic apparatus may further include: an illumination systemconfigured to condition a radiation beam, the radiation beam includingthe first wavelength of radiation, the second wavelength of radiation,or the first wavelength of radiation and the second wavelength ofradiation; a support constructed to support a patterning device, thepatterning device being capable of imparting the radiation beam with apattern in its cross-section to form a patterned radiation beam; asubstrate table constructed to hold a substrate; and a projection systemconfigured to project the patterned radiation beam onto a target portionof the substrate.

According to a sixth aspect of the present invention, there is provideda radiation source provided with a spectral purity filter or spectralpurity arrangement according to the first, second, third or fourthaspects of the present invention.

According to a seventh aspect of the present invention, there isprovided a method of affecting the spectral purity of a radiation beam,the radiation beam including a first wavelength of radiation and asecond wavelength of radiation, the method including: directing theradiation beam at a spectral purity filter or spectral purityarrangement according to the first, second, third or fourth aspects ofthe present invention.

According to an eighth aspect of the present invention, there isprovided a lithographic method, including: directing a radiation beamincluding a first wavelength of radiation and a second wavelength ofradiation at a spectral purity filter or spectral purity arrangementaccording to the first, second, third or fourth aspects of the presentinvention; and using radiation transmitted by the spectral purity filteror spectral purity arrangement to apply a pattern to a substrate coatedwith radiation sensitive material.

In the description of embodiments of the present invention, the terms‘substantially transparent’ and ‘substantially opaque’ have been used.‘Substantially transparent’ may be defined as a material or objecthaving a transmission of greater than 50% of the radiation in question,and in particular a transmission of between 80% and 100%. ‘Substantiallyopaque’ may be defined as a material or object having a transmission ofless than 50% of the radiation in question, and in particular atransmission of between 0% and 20%.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts.

FIG. 1 schematically depicts a lithographic apparatus according to anembodiment of the invention.

FIG. 2 is a more detailed but schematic depiction of the lithographicapparatus shown in FIG. 1.

FIG. 3 schematically depicts a spectral purity filter in accordance witha first embodiment of the present invention.

FIG. 4 schematically depicts a spectral purity filter in accordance withanother embodiment of the present invention.

FIG. 5 is a graph schematically depicting operating principlesassociated with the embodiments shown in FIGS. 3 and 4.

FIG. 6 is a graph schematically depicting further operating principlesassociated with the embodiments shown in and described with reference toFIGS. 3 and 4.

FIG. 7 schematically depicts yet further operating principles associatedwith the embodiments shown in FIGS. 3 and 4.

FIG. 8 schematically depicts the coating of side walls of apertures of aspectral purity filter in accordance with another embodiment of thepresent invention.

FIG. 9 schematically depicts a phase grating spectral purity filter inaccordance with another embodiment of the present invention.

FIG. 10 schematically depicts operating principles associated with thephase grating spectral purity filter shown in FIG. 9.

FIG. 11 schematically depicts further operating principles associatedwith the phase grating spectral purity filter shown in FIG. 9.

FIG. 12 schematically depicts yet further operating principlesassociated with the phase grating spectral purity filter shown in FIG.9.

FIG. 13 is a graph schematically depicting further operating principlesassociated with the embodiments shown in and described with reference toFIG. 9.

FIG. 14 schematically depicts and summarizes operating principlesassociated with the embodiments of FIGS. 9 to 13.

FIG. 15 schematically depicts an application of the embodiments shown inand described with reference to FIGS. 9 to 14.

FIG. 16 schematically depicts a spectral purity filter in accordancewith yet another embodiment of the present invention.

DETAILED DESCRIPTION

FIG. 1 schematically depicts a lithographic apparatus 2 according to oneembodiment of the invention. The apparatus 2 includes: an illuminationsystem (illuminator) IL configured to condition a radiation beam B(e.g., EUV radiation); a support structure (e.g., a mask table) MTconstructed to support a patterning device (e.g., a mask) MA andconnected to a first positioner PM configured to accurately position thepatterning device in accordance with certain parameters; a substratetable (e.g., a wafer table) WT constructed to hold a substrate (e.g., aresist-coated wafer) W and connected to a second positioner PWconfigured to accurately position the substrate in accordance withcertain parameters; and a projection system (e.g., a refractiveprojection lens system) PS configured to project a pattern imparted tothe radiation beam B by a patterning device MA onto a target portion C(e.g., including one or more dies) of the substrate W.

The illumination system may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic, electrostaticor other types of optical components, or any combination thereof, fordirecting, shaping, or controlling radiation.

The support structure supports, i.e., bears the weight of, thepatterning device. It holds the patterning device in a manner thatdepends on the orientation of the patterning device, the design of thelithographic apparatus 2, and other conditions, such as for examplewhether or not the patterning device is held in a vacuum environment.The support structure can use mechanical, vacuum, electrostatic or otherclamping techniques to hold the patterning device. The support structuremay be a frame or a table, for example, which may be fixed or movable asrequired. The support structure may ensure that the patterning device isat a desired position, for example with respect to the projectionsystem. Any use of the terms “reticle” or “mask” herein may beconsidered synonymous with the more general term “patterning device.”

The term “patterning device” used herein should be broadly interpretedas referring to any device that can be used to impart a radiation beamwith a pattern in its cross-section such as to create a pattern in atarget portion of the substrate. It should be noted that the patternimparted to the radiation beam may not exactly correspond to the desiredpattern in the target portion of the substrate, for example if thepattern includes phase-shifting features or so called assist features.Generally, the pattern imparted to the radiation beam will correspond toa particular functional layer in a device being created in the targetportion, such as an integrated circuit.

Examples of patterning devices include masks and programmable mirrorarrays. Masks are well known in lithography, and typically in an EUVradiation (or beyond EUV) lithographic apparatus would be reflective. Anexample of a programmable mirror array employs a matrix arrangement ofsmall mirrors, each of which can be individually tilted so as to reflectan incoming radiation beam in different directions. The tilted mirrorsimpart a pattern in a radiation beam which is reflected by the mirrormatrix.

The term “projection system” used herein should be broadly interpretedas encompassing any type of projection system. Usually, in an EUV (orbeyond EUV) radiation lithographic apparatus the optical elements willbe reflective. However, other types of optical element may be used. Theoptical elements may be in a vacuum. Any use of the term “projectionlens” herein may be considered as synonymous with the more general term“projection system”.

As here depicted, the apparatus 2 is of a reflective type (e.g.,employing a reflective mask).

The lithographic apparatus may be of a type having two (dual stage) ormore substrate tables (and/or two or more mask tables). In such“multiple stage” machines, the additional tables may be used inparallel, or preparatory steps may be carried out on one or more tableswhile one or more other tables are being used for exposure.

Referring to FIG. 1, the illuminator IL receives a radiation beam from aradiation source SO. The source and the lithographic apparatus may beseparate entities. In such cases, the source is not considered to formpart of the lithographic apparatus and the radiation beam is passed fromthe source SO to the illuminator IL with the aid of a beam deliverysystem including, for example, suitable directing mirrors and/or a beamexpander. In other cases the source may be an integral part of thelithographic apparatus. The source SO and the illuminator IL, togetherwith the beam delivery system if required, may be referred to as aradiation system.

The illuminator IL may include an adjuster for adjusting the angularintensity distribution of the radiation beam. Generally, at least theouter and/or inner radial extent (commonly referred to as σ-outer andσ-inner, respectively) of the intensity distribution in a pupil plane ofthe illuminator can be adjusted. In addition, the illuminator IL mayinclude various other components, such as an integrator and a condenser.The illuminator IL may be used to condition the radiation beam B to havea desired uniformity and intensity distribution in its cross-section.

The radiation beam B is incident on the patterning device (e.g., maskMA), which is held on the support structure (e.g., mask table MT), andis patterned by the patterning device. Having been reflected by the maskMA, the radiation beam B passes through the projection system PS, whichfocuses the beam onto a target portion C of the substrate W. With theaid of the second positioner PW and position sensor IF2 (e.g., aninterferometric device, linear encoder or capacitive sensor), thesubstrate table WT can be moved accurately, e.g., so as to positiondifferent target portions C in the path of the radiation beam B.Similarly, the first positioner PM and another position sensor IF1 canbe used to accurately position the mask MA with respect to the path ofthe radiation beam B, e.g., after mechanical retrieval from a masklibrary, or during a scan. In general, movement of the mask table MT maybe realized with the aid of a long-stroke module (coarse positioning)and a short-stroke module (fine positioning), which form part of thefirst positioner PM. Similarly, movement of the substrate table WT maybe realized using a long-stroke module and a short-stroke module, whichform part of the second positioner PW. In the case of a stepper (asopposed to a scanner) the mask table MT may be connected to ashort-stroke actuator only, or may be fixed. Mask MA and substrate W maybe aligned using mask alignment marks M1, M2 and substrate alignmentmarks P1, P2. Although the substrate alignment marks as illustratedoccupy dedicated target portions, they may be located in spaces betweentarget portions (these are known as scribe-lane alignment marks).Similarly, in situations in which more than one die is provided on themask MA, the mask alignment marks may be located between the dies.

The depicted apparatus 2 could be used in at least one of the followingmodes.

1. In step mode, the mask table MT and the substrate table WT are keptessentially stationary, while an entire pattern imparted to theradiation beam is projected onto a target portion C at one time (i.e., asingle static exposure). The substrate table WT is then shifted in the Xand/or Y direction so that a different target portion C can be exposed.In step mode, the maximum size of the exposure field limits the size ofthe target portion C imaged in a single static exposure.

2. In scan mode, the mask table MT and the substrate table WT arescanned synchronously while a pattern imparted to the radiation beam isprojected onto a target portion C (i.e., a single dynamic exposure). Thevelocity and direction of the substrate table WT relative to the masktable MT may be determined by the (de-)magnification and image reversalcharacteristics of the projection system PS. In scan mode, the maximumsize of the exposure field limits the width (in the non-scanningdirection) of the target portion in a single dynamic exposure, whereasthe length of the scanning motion determines the height (in the scanningdirection) of the target portion.

3. In another mode, the mask table MT is kept essentially stationaryholding a programmable patterning device, and the substrate table WT ismoved or scanned while a pattern imparted to the radiation beam isprojected onto a target portion C. In this mode, generally a pulsedradiation source is employed and the programmable patterning device isupdated as required after each movement of the substrate table WT or inbetween successive radiation pulses during a scan. This mode ofoperation can be readily applied to maskless lithography that utilizes aprogrammable patterning device, such as a programmable mirror array of atype as referred to above.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

FIG. 2 shows the lithographic apparatus 2 in more detail, including aradiation source SO, an illuminator IL (sometimes referred to as anillumination system), and the projection system PS. The radiation sourceSO includes a radiation emitter 4 which may include a discharge plasma.EUV radiation may be produced by a gas or vapor, such as Xe gas or Livapor in which very hot plasma is created to emit radiation in the EUVradiation range of the electromagnetic spectrum. The very hot plasma iscreated by causing partially ionized plasma of an electrical dischargeto collapse onto an optical axis 6. Partial pressures of e.g., 10 Pa ofXe or Li vapor or any other suitable gas or vapor may be required forefficient generation of the radiation. In some embodiments, tin may beused. FIG. 2 illustrates a discharge produced plasma (DPP) radiationsource SO. It will be appreciated that other sources may be used, suchas for example a laser produced plasma (LPP) radiation source.

The radiation emitted by radiation emitter 4 is passed from a sourcechamber 8 into a collector chamber 10. The collector chamber 10 includesa contamination trap 12 and grazing incidence collector 14 (shownschematically as a rectangle). Radiation allowed to pass through thecollector 14 is reflected off a grating spectral filter 16 to be focusedin a virtual source point 18 at an aperture 20 in the collector chamber10. Before passing through the aperture 20, the radiation passes througha spectral purity filter SPF. The spectral purity filter SPF isdescribed in more detail below. From collector chamber 10, a beam ofradiation 21 is reflected in the illuminator IL via first and secondreflectors 22, 24 onto a reticle or mask MA positioned on reticule ormask table MT. A patterned beam of radiation 26 is formed which isimaged in projection system PS via first and second reflective elements28, 30 onto a substrate W held on a substrate table WT.

It will be appreciated that more or fewer elements than shown in FIG. 2may generally be present in the source SO, illumination system IL, andprojection system PS. For instance, in some embodiments the illuminationsystem IL and/or projection system PS may contain a greater or lessernumber of reflective elements or reflectors.

It is known to use a spectral purity filter in a lithographic apparatusto filter out undesirable (e.g., out-of-band) wavelength components of aradiation beam. For instance, it is known to provide a spectral purityfilter including one or more apertures. The diameter of each aperture ischosen such that it diffracts one or more undesirable wavelengths ofradiation, while allowing one or more different wavelengths of desirableradiation to pass through the apertures. For instance, the undesirableradiation may include infrared radiation, whereas the desirableradiation may include EUV or beyond EUV radiation.

Proposed spectral purity filters which include apertures (sometimesreferred to as aperture spectral purity filters) are provided withapertures having a diameter of up to 20 μm. The small diameter of theseapertures limits the manufacturing options which are available to formthese apertures, and also reduces the mechanical robustness of thespectral purity filter. In accordance with an embodiment of the presentinvention, a spectral purity filter is provided with one or moreapertures, the apertures having a diameter greater than 20 μm. Becausethe apertures are greater than 20 μm in diameter, they may be moreeasily provided in a spectral purity filter than apertures having adiameter lower than 20 μm. For instance, apertures having a diameter ofgreater than 20 μm may be provided using well established laser drillingapparatus and techniques. An aperture having a diameter larger than 20μm is suitable for suppressing, for example, the 10.6 μm out-of-bandinfrared radiation mentioned above. Increasingly larger diameterapertures may also suppress infrared radiation having a higherwavelength. The apertures may have, for example, a diameter which isgreater than 20 μm and less than or equal to 200 μm in order to, forexample, cause diffraction of and therefore suppress infrared radiation.

The use of apertures having a diameter greater than 20 μm has manyadvantages associated with it when compared with proposed spectralpurity filters with apertures having a diameter less than 20 μm. Forinstance, laser cutting (sometimes referred to as laser drilling) isvery suitable for providing one or more (e.g., an array) of holes inmetal plates and plates formed from other materials, in order to form anaperture spectral purity filter. The minimum diameter of currentlyavailable laser micro-machining systems (e.g., laser drilling apparatus)is about 20 μm, meaning that such micro-machining systems areparticularly suitable for use in conjunction with embodiments of thepresent invention. In contrast, such micro-machining systems are notsuited for use in proposed aperture spectral purity filters, where theapertures have a diameter lower than 20 μm. Another advantage is thefact that the mechanical robustness of the spectral purity filterincreases when apertures in the filter are greater than 20 μm indiameter. This is because the wall thickness between adjacent aperturesfor a given fill ratio (i.e., the ratio of aperture space to plate ornon-aperture space) is greater when the apertures are greater than 20 μmin diameter than it is when the apertures are less than 20 μm indiameter. This is especially advantageous when the spectral purityfilter must withstand large stress concentrations, for example toprevent crack propagation. Another advantage is that in a spectralpurity filter with apertures having a diameter greater than 20 μm, EUVoptical losses on the sidewalls of the aperture are smaller than withapertures of a smaller diameter (for the same given thickness ofspectral purity filter). Such losses may occur due to small misalignmentof the spectral purity filter or the apertures of the filter, or due tothickness variation in the spectral purity filter. Losses are greaterwhen a radiation beam incident on the spectral purity filter, forexample a beam of EUV radiation, is divergent. A yet further advantageof using apertures having a diameter greater than 20 μm is that it makesit easier to apply a coating to sidewalls of the apertures. For example,it may be desirable to apply an infrared absorbing and/oranti-reflection coating to the sidewalls in order to maximize thesuppression of the infrared radiation, or to provide a coating topromote EUV reflection and therefore transmission through the aperture.

The above advantages will now be described in more detail with referenceto specific embodiments of the present invention.

FIG. 3 schematically depicts a spectral purity filter SPF according to afirst embodiment of the present invention. The spectral purity filterSPF includes a plate 50 in which a periodic array of circular apertures52 is provided. The diameter D of the apertures 52 is selected such thata first wavelength of radiation to be suppressed is substantiallydiffracted at the entrance of each aperture 52, while radiation of asecond, shorter wavelength is transmitted through the apertures 52. Thediameter of the apertures 52 is greater than 20 μm. A diameter slightlygreater than 20 μm is particularly suited to the diffraction andsuppression of 10.6 μm infrared radiation, which is often generated bythe radiation sources of EUV lithographic apparatus. The apertures 52may have a diameter which is greater than 20 μm and less than or equalto 200 μm, in order to suppress by diffraction longer wavelengths ofinfrared radiation.

The plate 50 can be formed from any suitable material. The plate 50should be substantially opaque to the first wavelength or range ofwavelengths which the spectral purity filter SPF is designed tosuppress. For instance, the plate 50 may reflect or absorb the firstwavelength, for example a wavelength in the infrared range of theelectromagnetic spectrum. The plate 50 may also be substantially opaqueto one or more second wavelengths of radiation which the spectral purityfilter SPF is designed to transmit, for example a wavelength in EUVrange of the electromagnetic spectrum. However, the spectral purityfilter SPF can also be formed from a plate 50 which is substantiallytransparent to the one or more wavelengths which the spectral purityfilter SPF is designed to transmit. This may increase the transmittanceof the spectral purity filter with respect to the one or morewavelengths which the spectral purity filter SPF is designed totransmit. An example of a material which may form the plate 50 of aspectral purity filter SPF is a metal. Another example is a thin foilthat is substantially transparent to EUV radiation.

The apertures 52 in the spectral purity filter SPF are arranged in ahexagonal pattern. This embodiment gives the closest packing of circularapertures, and therefore the highest transmittance for the spectralpurity filter. However, other arrangements of the apertures are alsopossible, for example square, and rectangular or other periodic oraperiodic arrangements may be used. For instance, in the case of anaperiodic array, a random pattern may be employed.

FIG. 4 is a schematic depiction of a spectral purity filter SPF inaccordance with another embodiment of the present invention. In thisembodiment, it can be seen that apertures 54 are provided in thespectral purity filter SPF. The apertures 54 are not circular, but areinstead elongated slots or slits. It can be seen that a shorterdimension SD of the aperture 54 has a length of between greater than 20μm and less than or equal to 200 μm. A longer dimension LD of theapertures 54 may be any length. It will be appreciated that theelongated apertures 54 shown in FIG. 4 are only suitable for situationswhere the radiation which is incident upon them, and which it is desiredto diffract, is substantially polarized in a direction substantiallyparallel to the longer dimension LD.

FIG. 5 is a graph depicting the geometrical transmittance of thespectral purity filter of FIG. 3, and how this varies as a function ofthe ratio of the wall thickness between apertures to the diameter ofthose apertures (or, in other words, holes in the spectral purityfilter). The geometrical transmittance is proportional to the area whichthe apertures in total define when the spectral purity filter is viewedend-on, as shown in FIG. 3. It can be seen from FIG. 5 that for atypical wall thickness of one-tenth of the aperture diameter, thegeometrical transmittance of the spectral purity filter is about 75%. Itcan be seen that the geometrical transmittance can be further increasedby increasing the diameter of the apertures, although this will probablylead to a less robust spectral purity filter due to a reduction in thewall thickness between apertures.

The minimum thickness of the spectral purity filter will depend onwhether it is desired to absorb the diffracted radiation in thesidewalls of the apertures of the spectral purity filter, or whether itis desired to absorb the diffracted radiation downstream of the spectralpurity filter. When the spectral purity filter is sufficiently thick,most of the diffracted radiation may be absorbed in the sidewalls of theapertures. Since only a small fraction of the incident power (of theradiation beam) reaches the exit of the apertures, the effect ofinterference between different apertures may be neglected. Consequently,the suppression of infrared radiation is approximately equal to that ofa single aperture of the same dimensions.

In the Fraunhofer (far field) approximation of diffraction from a singlecircular aperture, the intensity distribution as a function of thediffraction angle θ is given by:

${I(\theta)} = {I_{0}\left( \frac{2{J_{1}\left( {k\; a\;\sin\;\theta} \right)}}{k\; a\;\sin\;\theta} \right)}^{2}$

where k=2π/λ is the wavenumber, a is the aperture radius and J₁ is thefirst-order Bessel function of the first kind.

It is desirable to suppress the transmission of infrared radiation, asdiscussed above. Depending on the application in question, it may bedesirable to suppress the infrared radiation by a factor of 100 or more,or in other applications by a factor of around 10. In some applications,a suppression factor of 20 may be desirable. This means that only 5% ofthe infrared radiation incident on the spectral purity filter will betransmitted by the spectral purity filter.

FIG. 6 shows the angle within which 5% of the infrared radiation (interms of power, or in other words intensity) is contained as a functionof the aperture diameter (where the aperture diameter is twice theaperture radius, i.e., 2a). FIG. 6 shows that if apertures having adiameter of 30 μm are used, the angle containing 5% of the transmittedradiation will be 2.9°. In order to achieve the desired suppressionfactor of 20 (in other words, to achieve a transmission of 5% of theincident infrared radiation), all radiation diffracted by more than 2.9°must be absorbed. Such absorption can take place within the spectralpurity filter. The minimum thickness of the spectral purity filter isthen determined by the aperture diameter and the minimum diffractionangle outside which all radiation is to be absorbed. For example, withthe 30 μm diameter apertures described above, the angle containing 5% ofthe transmitted radiation was described as being 2.9°. Therefore, inorder to ensure that all radiation outside of this diffraction angle isabsorbed by the sidewalls of the apertures, the spectral purity filtermust have a minimum thickness of:30 μm/tan 2.9°=0.584 mm

More infrared radiation having a wavelength of 10.6 μm can be suppressedby increasing the thickness of the spectral purity filter. However, thetransmittance of radiation having a shorter wavelength (e.g., EUVradiation) will be reduced due to the higher aspect ratio of theapertures.

For a thinner spectral purity filter, a substantial fraction of theincident power is transmitted through the spectral purity filter. Thedegree of interference between the radiation transmitted through theapertures depends on the coherence of the radiation. For example, whenthe incoming radiation is very incoherent, there is no substantialinterference between the apertures and the diffraction pattern isdescribed by the single-aperture approximation described above. In thiscase, it is possible to absorb the infrared radiation at a locationbehind (i.e., downstream of) the spectral purity filter. For example, ifthe spectral purity filter is located 0.5 m before an intermediate focusof the lithographic apparatus (for example, the virtual source point 18shown in and described with reference to FIG. 2), the diffractedradiation can be absorbed by a plate or the like located at theintermediate focus. The plate is provided with an aperture having adiameter of 8 mm for transmitting, for example, EUV radiation. In thiscase, all radiation diffracted by 0.9° is prevented from passing throughthe aperture. From FIG. 6, it can be seen that the apertures in thespectral purity filter can have a diameter as large as 100 μm in orderto ensure that the angle containing 5% of the transmitted radiation is0.9° (or in other words to ensure that the suppression factor isapproximately 20). Since the spectral purity filter in this embodimentwould only act to diffract the radiation, and not to absorb it, thespectral purity filter may be as thin as minimally required formechanical stability and may be, for example, 0.1 mm thick.

In FIGS. 3 and 4, the apertures of the spectral purity filters have beenshown and described as being part of a periodic array of apertures. Acoherent source of infrared radiation incident upon the spectral purityfilter combined with a periodic aperture array could give rise to a farfield diffraction pattern with a very sharp (e.g., less than 0.5°)central maximum, in which 80-90% of the infrared radiation intensity iscontained. If a transmission profile having such a far field diffractionpattern were to be established, it would be difficult or impossible toseparate the infrared radiation from the EUV radiation in the far field.Therefore, in the case of a substantially coherent source of radiation,other methods of affecting the diffraction pattern may be used. Forexample, an aperiodic array of apertures will ensure that the centralmaximum of the resultant diffraction pattern is broadened, and is notvery sharp. This would make it easier to separate the infrared radiationfrom the EUV radiation in the far field, for example using a plateprovided with an aperture as described above.

FIG. 7 schematically depicts and summarizes the embodiments shown in anddescribed with reference to FIGS. 3 to 6. A radiation beam 60 includingEUV radiation 62 and infrared radiation 64 is incident upon a spectralpurity filter SPF as described above in relation to, for example, FIG.3. Downstream of the spectral purity filter SPF is located a plate 66which is capable of absorbing infrared radiation 64. The plate 66 isprovided with an aperture 70, and is located adjacent to an intermediatefocus 68 of the radiation beam 60 such that the intermediate focus 68 islocated in the aperture 70. When the radiation beam 60 is incident uponthe spectral purity filter SPF, the EUV part of the radiation beam 62passes through the spectral purity filter SPF without being diffracted.At the same time, the infrared part of the radiation beam 64 isdiffracted by apertures of the spectral purity filter SPF, as describedabove. Thus, at the location of the plate 66, it can be seen that theEUV part of the radiation beam 62 passes through the aperture 70,whereas a majority of the diffracted infrared part of the radiation beam64 is instead directed toward and absorbed by the plate 66. Downstreamof the plate 66, it will be appreciated that the infrared part 64 of theradiation beam has been reduced or substantially eliminated, leaving aradiation beam 72 which includes substantially EUV radiation.

In the above embodiments, the apertures have been described as beingcircular or slot or slit-like. The apertures can be other shapes, suchas for example rectangular, hexagonal, pentagonal, etc.

The spectral purity filter described above has been described as beingformed from a plate. The spectral purity filters described herein caninstead be formed from a mesh, or two overlapping perpendicular wiregrids. It may be easier to manufacture a mesh than it is to drill holesin a plate. Wire grids may also be easier to manufacture than an arrayof circular apertures in a plate. For instance, each of the wire gridscan be formed, for example, using laser interference lithography. Thismay be quicker and easier to undertake than providing a spectral purityfilter with an array of drilled apertures, where each aperture may needto be individually drilled using a laser machine tool.

In the above embodiments, absorption in the sidewalls of apertures ofthe spectral purity filter has been described. The spectral purityfilter may be formed from a material which is suitable for absorption ofone or more wavelengths of radiation. For example, the spectral purityfilter may be formed from a material with a high infrared radiationabsorptivity. For instance, many glasses (e.g., fused silica) andceramics (e.g., TiO₂) fall within this category. As mentioned above, theuse of apertures having diameters greater than 20 nm makes it easier toapply a coating to sidewalls of the apertures. This means that thesidewalls of the apertures may be provided with a coating which is orincludes a material which absorbs or inhibits reflection of certainwavelengths of radiation. For example, in the case of the absorption ofinfrared radiation, coatings may be provided which are formed from orinclude materials such as the glasses or ceramics mentioned above.Alternatively or additionally, an anti-reflection coating may be appliedto the sidewalls of the apertures, including for example CO₂ laserwindow materials, such as ZnSe, ZnS, GaAs and Ge, and/or low refractiveindex halides such as ThF₄ and YF₃. The coating may additionally oralternatively promote the reflection of one or more wavelengths ofradiation, such as for example EUV radiation.

FIG. 8 shows an embodiment of a spectral purity filter. In thisembodiment, a coating (as described above) has been applied to sidewallsof the apertures of the spectral purity filter. FIG. 8 shows a spectralpurity filter SPF in section view. Apertures 80 are shown as beingprovided in a plate 81 of the spectral purity filter SPF. Sidewalls ofthe apertures 80 have been provided with an infrared radiation absorbingmaterial 82.

EUV radiation 84 and infrared radiation 86 are shown as being incidentupon the spectral purity filter SPF and apertures 80 of the spectralpurity filter SPF. The infrared radiation 86 is diffracted as it entersthe apertures 80, and is absorbed by the coating 82 on the side walls ofthe apertures 80. It can be seen that radiation transmitted by thespectral purity filter SPF includes mainly EUV radiation 84.

The coating 82 provided on the sidewalls of the apertures 80 does notneed to promote or suppress reflection of one or more wavelengths ofradiation. The coating 82 can instead be used to prevent the apertures80 from degradation or environmental damage.

The spectral purity filters of above embodiments have been described asbeing formed from plates (or the like) that are substantially opaque toan undesired wavelength of radiation, for example infrared radiation.The spectral purity filters described above are provided with apertureswhich diffract the undesired wavelength of radiation, while allowing adesired wavelength of radiation to be transmitted by the apertures.Thus, the spectral purity filters described above will reflect or absorbundesired wavelengths of radiation, either in the side walls of theapertures or in or from parts of the spectral purity filters upon whichthe undesired wavelength is incident (for example, parts of the spectralpurity filter that do not define apertures). Absorption of the undesiredwavelength can cause the spectral purity filter to heat up. Such heatingcan result in damage to the spectral purity filter and/or distortion ofthe spectral purity filter. Such distortion or damage may detrimentallyaffect the functionality of the spectral purity filter.

For the reasons given above, in some applications it may therefore bedesirable to reduce the heat load on a spectral purity filter. Accordingto an embodiment of the present invention, this may be achieved by theuse of a spectral purity filter that acts as a phase grating for theundesired wavelength or wavelengths of radiation. The phase gratingspectral purity filter includes a material that is substantiallytransparent (and, ideally, fully transparent) to the undesiredradiation. Furthermore, the phase grating spectral purity filter isprovided with apertures which are constructed (e.g., have a spacing ordiameter which is) such that most of the undesired radiation isdiffracted into diffraction orders other than the zero order. Oneadvantage of this solution is that the undesired radiation is notabsorbed by the special purity filter, and does therefore not heat thespectral purity filter. Another advantage is that, since most of theundesired radiation is not diffracted into the zero order, a structureprovided with an aperture placed at a location aligned with the zeroorder will allow desired (and not diffracted) radiation to pass throughthe aperture, while the undesired radiation is blocked by material(e.g., a plate) surrounding the aperture. In the case of an aperiodicarray of apertures, there may not be any ‘orders’ as such. Therefore,according to a more general embodiment of the present invention, thespacing or diameter of the one or more apertures of the spectral purityfilter may be configured to ensure that less than 50% of radiation of afirst wavelength of radiation (e.g., infrared radiation) is able to passthrough the further aperture that is provided in the structure. Thespacing or diameter of the one or more apertures of the spectral purityfilter may be configured to ensure that less than 10% of radiation of afirst wavelength of radiation (e.g., infrared radiation) is able to passthrough the further aperture that is provided in the structure. Thespacing or diameter of the one or more apertures of the spectral purityfilter may be configured to ensure that less than 5% of radiation of afirst wavelength of radiation (e.g., infrared radiation) is able to passthrough the further aperture that is provided in the structure

FIG. 9 schematically depicts a phase grating spectral purity filterPGSPF in accordance with an embodiment of the present invention. Thephase grating spectral purity filter PGSPF includes a plate 90 that issubstantially transparent to an undesired (e.g., ‘first’) wavelength ofradiation. For instance, if the undesired radiation includes 10.6 μmradiation, the plate 90 may be formed from silicon, ZnSe, ZnS, GaAs, Ge,diamond or diamond-like carbon.

The plate 90 is provided with an array of apertures 92. The aperturediameter AD is chosen such that the undesired wavelength of radiationdiffracts when it is incident upon the aperture 92. For instance, theapertures 92 may conveniently be greater than 20 μm in diameter, forease of manufacturing as described above. The array of apertures 92 havea periodicity PE which is chosen such that the first diffraction orderof the diffracted undesired wavelength of radiation is substantiallyseparated from the zero diffraction order. The significance of thisarrangement will be described in more detail below. Typically, theperiodicity PE is of the same order of magnitude as the wavelength ofradiation that the phase grating spectral purity filter PGSPF isdesigned to diffract. However, in other embodiments, the periodicity PEmay be up to two orders of magnitude larger than this wavelength (forexample, if only a very small degree of diffraction of the undesiredwavelength of radiation is required).

So far FIG. 9 has been described in relation to the diffraction of anundesired wavelength of radiation as a consequence of the incorporationof the apertures 92. As mentioned above, this spectral purity filter isa phase grating spectral purity filter PGSPF. The significance of the“phase grating” prefix will now be described.

FIG. 10 shows the phase grating spectral purity filter PGSPF in sectionview. FIG. 10 illustrates the apertures 92 and the material forming theplate 90 surrounding these apertures 92. Undesired radiation 94 (e.g.,infrared radiation) is shown as being incident upon the phase gratingspectral purity filter PGSPF. The phase grating spectral purity filterPGSPF has a thickness H which is chosen such that the phase differencebetween the radiation passing through the plate 96 and the radiationpassing through the apertures is π radians. In other words, radiationemerging from the apertures 98 is 180° out of phase with radiationemerging from the plate 96. If it is assumed that the diameter of theapertures is substantially larger than the wavelength of undesiredradiation, diffraction effects can be assumed to be small and thereforethe phase shift can be calculated based on a plane wave front.

The phase difference Δφ between radiation passing through the plate andradiation passing through the apertures is thus given by:

${\Delta\;\varphi} = \frac{2\pi\;{H\left( {n_{1} - n_{0}} \right)}}{\lambda}$

where n₁ is the refractive index of the material of the plate and n₀=1is the refractive index of vacuum.

Thus, the thickness that gives a phase difference of π is given by:

$H = {\left( {m + \frac{1}{2}} \right)\frac{\lambda}{n_{1} - n_{0}}}$

where m=0, 1, 2, . . . is a non-negative integer.

When silicon is chosen to form the material or the plates 90, thethickness H (in μm) that gives a phase difference of π is given by:H=(2.19+m 4.38) μm.

As is known in the art, when the thickness of the phase grating spectralpurity filter is such that the phase difference between radiationpassing through the plate and radiation passing through the apertures isπ radians, destructive interference of the undesired wavelength ofradiation will take place.

FIGS. 11 and 12 depict the effects of the phase grating spectral purityfilter of FIGS. 9 and 10 on an incoming beam of radiation including anundesired wavelength of radiation. An undesired wavelength of radiation94 is shown as being incident upon the phase grating spectral purityfilter PGSPF. The diameter and periodicity of apertures of the phasegrating spectral purity filter PGSPF are chosen such that the undesiredwavelength of radiation is diffracted, and diffracted such that a firstdiffraction order 100 is substantially separated from a zero diffractionorder 102. FIG. 11 does not, however, represent the effects on the zeroorder 102 of the destructive interference mentioned above.

FIG. 12 more accurately represents the cumulative effects of theseparation of the first diffraction order 100 from the zero diffractionorder 102, and the effects on the zero diffraction of the destructiveinterference mentioned above. The zero diffraction order 102 containsonly a small fraction of the incident intensity of the undesiredwavelength of radiation 94 (typically less than 10%). Most of theundesired radiation is diffracted into higher orders, such as the firstorder and higher orders. To ensure that most of the incident intensityof the undesired wavelength of radiation 94 is diffracted into higherdiffraction orders, the fill factor of the phase grating spectral purityfilter PGSPF should have a fill factor of the order of 50% or greater(the fill factor being defined as the fractional area occupied by thematerial forming the plate, and not the spaces left by the apertures).

A plate 104 may be provided to absorb the first and higher diffractionorders. An aperture 106 is provided in the plate 104 to allow, forexample, undiffracted radiation to pass through (for example EUVradiation). It can be seen that the zero diffraction order 102 is alsoable to pass through the aperture, although this order (as describedabove) will typically be less than 10% of incident intensity. The phasegrating spectral purity filter PGSPF and the plate 104 and aperture 106therefore prevent at least 90% of the undesired radiation from passingany further, for example into or through a part of a lithographicapparatus.

Because the fill factor is 50%, this means that the transmittance ofradiation which is blocked by the plate (for instance, EUV radiation) isalso of the order of 50%. The transmittance of radiation not blocked bythe plate can be increased by reducing the fill factor, but this resultsin a reduced suppression of the undesired radiation (for example,infrared radiation).

FIG. 13 shows the increase in infrared radiation transmittance of thephase grating spectral purity filter described above for an EUVtransmittance increase from 50% to 100% (which corresponds to the fillfactor being reduced from 50% to 0%). The graph represents arelationship for a one-dimensional phase grating, for instance a gratingincluding a single row or column of apertures. It can be seen that, forexample, at an EUV transmittance of 60%, the infrared transmittance is4%, and thus the relative suppression factor of infrared radiation is 15(i.e., the filter decreases the ratio of infrared radiation to EUVradiation by a factor of 15).

FIG. 14 schematically summarizes the principles discussed in relation toFIGS. 9 to 13. A beam of radiation including an EUV component 108 and aninfrared component 110 is incident upon a phase grating spectral purityfilter PGSPF as described above. The phase grating spectral purityfilter PGSPF is shown in relation to a plate 112 provided with anaperture 114. It can be seen that the EUV component 108 passes throughapertures (not shown in the Figure) of the phase grating spectral purityfilter PGSPF and towards and through the aperture 114 in the plate 112.In contrast, the infrared component 110 is destructively interferedwith, as well as being and diffracted by the phase grating spectralpurity filter PGSPF. The result is that the first diffraction order ofthe infrared component 110 is separated from the zero order to such anextent that the first and higher diffraction orders may be blocked bythe plate 112, and prevented, for example, from passing on to andthrough a lithographic apparatus. The zero diffraction order issubstantially reduced or eliminated.

FIG. 15 schematically depicts an embodiment. The phase grating spectralpurity filter PGSPF as described above is placed in front of (i.e.,up-stream of) an intermediate focus 120 of a lithographic apparatus, forexample the virtual source point 18 shown in and described withreference to FIG. 2. Referring back to FIG. 15, the intermediate focus120 is located within an aperture 122 of a plate 124. The plate may, forexample, form part of a source or illuminator housing. A radiation beam126 incident upon the phase grating spectral purity filter PGSPFincludes an EUV component 128 and an infrared component 130. Asdescribed above, the phase grating spectral purity filter PGSPF isconfigured to diffract and cause destructive interference of theinfrared component 130 such that the majority of the infrared component130 is either destructively interfered with, or diffracted to such anextent that is not able to pass through the aperture 122. For instance,it can be seen that the first and higher diffraction orders 132 of theinfrared component 130 are diffracted such that they are unable to passthrough the aperture 122. A small percentage of the zero diffractionorder 134 of the infrared component 130 is able to pass through theaperture 122. However, this small percentage of the zero diffractionorder 134 will be only a fraction of the infrared component 130, andmaybe for example, less than 10% of the incident intensity.

A typical diameter of the aperture 122 at the intermediate focus 120(for example, the entrance pupil of an illuminator) is 8 mm. Thus, ifthe distance from the phase grating spectral purity filter PGSPF to theintermediate focus 120 is 0.1 m, diffraction orders separated by morethan 2.3° (arctan (0.004/0.1)) from the zero order are suppressed. Forthe first diffraction order (and therefore higher diffraction orders) tobe diffracted by more than 2.3°, the periodicity of the apertures of thephase grating spectral purity filter PGSPF referred to above should beless than 264 μm (calculated using λ/sin θ=264 μm, where λ=10.6 μm andθ=2.3°).

In the embodiments described above in relation to FIGS. 9 to 15, up to50% of the EUV radiation is blocked by the material which forms theplate of the phase grating spectral purity filter. It is desirable toincrease the EUV transmittance whilst still suppressing the infraredradiation by destructive interference and diffraction. In accordancewith a further embodiment of the present invention, material which formsthe phase grating spectral purity filter contains a first array ofapertures, and a second array of apertures, the second array ofapertures being distributed around the first array of apertures andhaving diameters which are less than those of the first array. The firstarray of apertures have a diameter sufficient to cause diffraction ofradiation which is to be suppressed (e.g., infrared radiation). Thesecond (or further) array of apertures have a diameter which is lessthan the wavelength of the radiation which it is desired to suppress.This means that the second array of apertures (which may be referred toas sub-wavelength apertures) do not affect the diffraction of theradiation which it is desired to suppress, but will allow more shorterwavelength (for example EUV) radiation to pass through the phase gratingspectral purity filter.

For the purposes of determining phase differences for the radiation thatpasses through the apertures and the material of the phase gratingspectral purity filter PGSPF, etc., the refractive index of the materialwith the sub-wavelength apertures may be approximated by a so-calledeffective medium approximation, for example, based on a weighted averageof the dielectric constants of the material and vacuum (if the phasegrating spectral purity filter is used in a vacuum). Therefore, in orderto take into account the incorporation of the sub-wavelength apertures,the thickness of the phase grating spectral purity filter must bechanged accordingly so as to still obtain the desired phase shift andsubsequent destructive interference of the undesired radiation whichpasses through the larger apertures and through the material forming thephase grating spectral purity filter.

FIG. 16 illustrates an embodiment of a phase grating spectral purityfilter which is provided with sub-wavelength apertures. FIG. 16schematically depicts a phase grating spectral purity filter PGSPF.Material 144 forming the phase grating spectral purity filter PGSPF isprovided with a first array of apertures 140. The first array ofapertures is arranged (e.g., have a diameter which is sufficient) tocause slight diffraction of a first wavelength of radiation, for examplean undesired wavelength of radiation such as, for example, infraredradiation. The first array of apertures is also arranged (e.g., have adiameter which is sufficient) to allow transmission of a secondwavelength of radiation, for example a desired wavelength of radiationsuch as, for example, EUV radiation. The second wavelength of radiationhas a shorter wavelength that that of the first wavelength of radiation.

A second (or in other words further) array of apertures 142 is alsoprovided in material 144 forming the phase grating spectral purityfilter PGSPF. The second array of apertures 142 have a diameter which isless than the diameter of the apertures 140 forming the first array. Thediameter of the apertures of the second array 142 is less than the firstwavelength of radiation of which the phase grating spectral purityfilter is arranged to diffract and cause destructive interference.Preferably the diameter of the apertures of the second array 142 is lessthan half of the wavelength of the first wavelength of radiation, inorder to ensure the validity of an effective medium approximation asdescribed above. The diameter of the apertures of the second array 142is greater than the second wavelength of radiation which the phasegrating spectral purity filter is arranged to transmit. This means thatthe diffraction of the first wavelength of radiation (e.g., infraredradiation) is not affected, while the transmission of the secondwavelength of radiation (e.g., EUV radiation) is increased by provisionof the further array of apertures.

A silicon phase grating spectral purity filter according to thisembodiment of the present invention could include a two dimensionalarray of larger apertures with a diameter of 100 μm and a fill factor of50%. The material (i.e., silicon) between the large apertures could beprovided with a further array of smaller diameter apertures with adiameter of 4 μm and, for example, a fill factor of 50%. The effectiverefractive index of the material between the large apertures iscalculated according to a weighted average of the dielectric constant(which is approximately equal to the square of the refractive index)resulting in a value of √(0.5×3.42²+0.5×1²)=2.52. Consequently, in orderto achieve a phase shift of it of the first wavelength of radiation asit passes through the phase grating spectral purity filter PGSPF (toachieve destructive interference), the calculation that needs to beperformed to determine the thickness of the phase grating spectralpurity filter PGSPF will need to be modified to take into account thiseffective refractive index. The thickness of the silicon phase gratingspectral purity filter PGSPF should therefore be calculated using:H=(3.49+m 6.98) μm

where m is a positive integer or zero. For example, H may be 6.98 μm. Itwill be appreciated that this calculation will be different if amaterial having a refractive index different than that of silicon isused to form the phase grating spectral purity filter PGSPF.

In this phase grating spectral purity filter, the infrared transmittanceis close to zero (see for example the graph of FIG. 13). With an overallfill factor of 25%, the EUV transmittance of the phase grating spectralpurity filter is approximately 75%.

It will be appreciated that the use of sub-wavelength (with respect to awavelength to be diffracted by larger apertures of the spectral purityfilter) apertures is not restricted to use in phase grating spectralpurity filters. Such sub-wavelength apertures may be provided in anyspectral purity filter in order to increase the transmission of thespectral purity filter to one or more wavelengths of radiation.

In the above described embodiments, a ‘desired’ wavelength of radiationhas been described as being a wavelength of radiation in or below theEUV range of the electromagnetic spectrum. Furthermore, an ‘undesired’wavelength has been described as a wavelength of radiation in theinfrared part of the electromagnetic spectrum. It will be appreciatedthat the present invention is also applicable to other wavelengths ofradiation. For example, the embodiments described above in relation toFIGS. 9 to 15 (where the first diffraction order is separated from thezero order such that the first order does not pass through an apertureprovided downstream of the spectral purity filter) may also beapplicable to wavelengths of radiation other than EUV radiation andinfrared radiation. Similarly, the embodiments described above inrelation to FIG. 16 (where sub-wavelength apertures are used to improvethe transmittance of a phase grating spectral purity filter to a desiredwavelength of radiation) may be used in conjunction with wavelengths ofradiation other than EUV radiation.

Although the above description of embodiments of the invention relatesto a radiation source which generates EUV radiation (e.g., 5-20 nm), theinvention may also be embodied in a radiation source which generates‘beyond EUV’ radiation, that is radiation with a wavelength of less than10 nm. “Beyond EUV” radiation may for example have a wavelength of 6.7nm or 6.8 nm. A radiation source which generates “beyond EUV” radiationmay operate in the same manner as the radiation sources described above.The invention is also applicable to lithographic apparatus that uses anywavelength of radiation where it is desired to separate, extract,filter, etc. one or more wavelengths of radiation from another one ormore wavelengths of radiation. The described spectral purity filter maybe used, for example, in a lithographic apparatus or a radiation source(which may be for a lithographic apparatus). The invention may also beapplied to fields and apparatus used in fields other than lithography.

The description above is intended to be illustrative, not limiting.Thus, it will be apparent to one skilled in the art that modificationsmay be made to the invention as described without departing from thescope of the claims set out below.

What is claimed is:
 1. A spectral purity filter, comprising: a base having a thickness defined by two opposing major surfaces with a first array of apertures formed therein, the first array of apertures extending through the thickness of the base and defining a sidewall within each aperture; and a coating provided on the sidewalls of the first array of apertures, wherein the first array of apertures are configured to diffract a first wavelength of radiation and to allow at least a portion of a second wavelength of radiation to be transmitted, the second wavelength of radiation being shorter than the first wavelength of radiation, wherein the base further comprises a second array of apertures, each aperture of the second array of apertures having a diameter less than the first wavelength.
 2. The spectral purity filter of claim 1, wherein the coating is arranged to absorb at least a portion of the first wavelength of radiation.
 3. The spectral purity filter of claim 1, wherein the coating is arranged to inhibit reflection of at least a portion of the first wavelength of radiation.
 4. The spectral purity filter of claim 1, wherein the coating is arranged to promote reflection of at least a portion of the second wavelength.
 5. The spectral purity filter of claim 1, wherein the coating is arranged to inhibit degradation or environmental damage to the first array of apertures.
 6. The spectral purity filter of claim 1, wherein the first array of apertures each have a diameter greater than 20 μm.
 7. The spectral purity filter of claim 1, wherein the diameter is less than half of the first wavelength.
 8. The spectral purity filter of claim 7, wherein the diameter is greater than the second wavelength.
 9. The spectral purity filter of claim 1, wherein the first array of apertures have a periodicity such that the first diffraction order of the first wavelength of radiation is substantially separated from the zero diffraction order.
 10. The spectral purity filter of claim 1, wherein the base comprises at least one of a plate, a mesh, and a wire grid.
 11. The spectral purity filter of claim 1, wherein the first array of apertures or the second array of apertures are circular apertures arranged in a periodic array.
 12. The spectral purity filter of claim 11, wherein the periodic array comprises a hexagonal pattern array.
 13. The spectral purity filter of claim 1, wherein the first array of apertures are elongated slots arranged in a linear array. 